Colored glazed coated granules and roofing sheet material surfaced therewith



Patented Oct. 12, 1943 COLORED GLAZED COATED. GRANULES AND ROOFING SHEETMATERIAL SURFACED George W. Swenson, St. Paul, Minn, assignor to 1Minnesota Mining & Manufacturing Company, St. Paul, Minn., a corporationof Delaware No Drawing. Application July 23, 1941,

Serial No. 403,758

22 Claims.

Thi invention relates to improved process for treating mineralsubstances and to the resultant product and more particularly relates tothe manufacture of glazed granules, more especially of the type coveredwith a glaze containing a coloring matter, and, while the invention isdirected particularly to the treatment of naturally refractory materialswith a colored glaze or the like, and the article per se, as well as aroofing material made with this'article, it will be understood that theinvention possesses a wide field of utility and may be adapted foranalogous purposes.

This application is a continuation-in-part of my copending applicationSerial No. 248,035, filed Dec. 28, 1938, which, in turn, is a divisionof my application Serial No. 647,966, filed December 19, 1932. It alsopossesses disclosure common to my "copending application Serial No.647,967, also filed December 19, 1932, as well as disclosure common tomy copending application Serial No. 717,851, filed March 28, 1934, and

hence may be termed a continuation-in-part also of said applications.

The product of this invention may be incorporated as a coating for aroofing shingle or as an aggregate for incorporation in artificial stonesuch as cast stone and the like, and for the base granule there may beemployed natural or artificial refractory substances such, for example,as quartz, quartzite, smoky quartz, cairngorm stone, sand, refractorysubstances; or, when coating temperatures and desired colors .willpermit, slate, crushed brick and other baked clay and similar material,porcelain, terra cotta or other ceramic waste or cement compositions maybe employed.

The finished product, that is to say, the natural or artificialrefractory material with a glazed coating may be incorporated as aweather coating in a bituminously coated sheet for forming roofingmaterial either in rolls or in shingles, or may be incorporated in caststone.

It is well recognized that many industries employing granular materialsare handicapped in the use of colored granules, inasmuch as the existingavailable material, such as naturally occurring colored rock, does notgive either a wide range of colors, nor are the colors of an attractiveor permanent nature.

Heretofore efforts havev been made to treat crushed mineral materialwith a color bound to the granule with an organic bond and this isunsuited for many uses, inasmuch as the colors are only relativelypermanent, and the organic bond does not have the life or ageingcharacteristics, particularly-where it is desired to use the granules inpermanent materials such as cast stone and the like. Furthermore, in thecase of cast stone, the organic bond on the granules prevents permanentadhesion betweenthe granule and the matrix of the cast stone.

As an alternative, inorganic bonds have been experimented with and chiefamong thoseknown to me are the type employing a sodium-silicate as thebase. In these methods and thearticles resulting therefrom the colorsare contained in a sodium silicate colloidal dispersion, and thegranules are then coated with this mixture. The

coated granulesare then fired to fix the color there are manydefects insuch granules which 1 limit their possible uses, included in thesedetreme permanence;

fects being the lack of strength of color and proper tone of color,inadequate weathering properties, and the limiting of the range ofcolors available to be produced. Furthermore, these methods known to meare not capable of being carried out at a reasonable cost.

The principal objects and advantages of this invention reside in theprovision of an improved article of manufacture, glaze coated granules,particularly of the colored type, which may be manufactured with a widerange of colors of exthe provision of treated granules of the characterreferred to in which a good bonding action can be attainedbet ween thefinished granulated material and other bonding materials; the provisionof an improved granulated material to be employed a a weather coating onvarious objects which possesses a permanence of color not heretoforeattained by the methods and in the articles known to me, and theprovision of glazed granules in which I may be enabled to obtain certaindesired color eiiects and to'accomplish this purpose at a cost less"lated material of a glaze having an affinity for v and an ability tocarry color in the necessary strength, and at the same time to'possesssuitable coeflicients of expansion and contraction so that the glaze maybe successfully employed on a wide range of difierent materials employedas the base granule.

This invention also has as one of its objects the provision of animproved method of preparing the improved glaze and applying the same tothe granulated material to produce the colored granule and which willproduce colored aggregates having high permanence of color and toaccomplish this at a relatively low cost; the provision of an improvedmethod of manufacturing colored granulated material in which theintensity of color may be regulated to a desired degree and to such anextent that the color and/or the glaze are not undesirably affected byany quantity of substance such as a fiuxing agent employed for enhancingthe reaction between the glaze and the granule to which it is applied;and the provision of an improved method of manufacturing coloredgranular material in which the glazing effect of the coating applied tothe individual granules is enhanced, even with the presence of a deepcolor in the glaze.

I have found that my improved methods of coating granulated materialspermits of the production of colored aggregates wherein the glazeapplied to the individual granules is one containing a coloring agent orpigment, and is substantlally opaque, without, however, producing adulling of the glaze, thus overcoming one of the principal disadvantagesof the prior colored granulated material heretofore produced either byartificial methods or that found in natural deposits.

For the purpose of my invention, I have preferred to employ quartzitegranules, inasmuch as quartzite is metamorphosed sand stone in whichquartz particles are cemented with a siliceou cement, sometimes, but notalways, containing calcium carbonate and iron oxide. The porosity ofquartzite is relatively low, due to the fact that the only possibleporosity of quartzite is produced by the intervening portions of cementbetween the quartz grains, which is a relatively small part of thegranule itself. Quartzite appears to possess a further characteristic ofhaving anaifinity for the glaze as applied thereto and particularly thequartzite granule appears to react on its outer surface with the glazecoating so that the coating is knitted firmly to the granule by the heattreatment which will be hereinafter described.

I am not prepared to state just what is the exact reaction between theimproved glaze and the preferred granulated material quartzite, but itappears that quartzite is partially dissolved by the molten glazecoating and reacts chemically with material of the glaze by furnishingthe sillca to form complex silicates, these silicates and othercompounds being mutually soluble, at least to great extent. It may beand undoubtedly is true that part of the material, such as in someinstances color, is inert to the glaze.

One method of producing quartzite granules may include the step sofemploying a glazing material in a powdered form ground in an 011grinding liquid such as fish oil, varnish makers gloss oil or the like,this powdered glaze containing the desired color.

It is now appropriate to point out that the proportion of color toglazing material may be at the outset controlled, and, for my purposes,I prefer to add a quantity of pigment to the mass of glaze to such anextent that the glaze has proper flowing properties, and the desiredcolor is obtained in a single step. The proportions of pigment to themass of glaze are controlled to an extent short of that amount whichwould dull the glaze when in a finished condition. To illustrate thisfeature of my invention it may be made clear that, in mixing commonpaints, it is possible to add so much pigment to the vehicle that,instead of leaving a glossy appearance when dry, as in the case of someunbaked enamels when dry, that the finished paint when applied to asurface is dull. This is due to an excess of pigment beyond the desiredand proper amount, and, in my present invention, I add to the glaze andoil vehicle, either before the glaze is mixed with the oilor afterwards,such amount of pigment as will produce in the finished article a deepcolor having a glossy surface appearance.

Where a gloss to the finished granule is not desirable, I add a mattingcompound to reduce the gloss. This compound does not rob the bond oflife as increasing the pigment content does to paint.

I prefer to use a gloss oil consisting of rosin dissolved in mineralspirits, as a vehicle for suspending and distributing the glaze,inasmuch as this is a low-cost deposit in a form adhering tenaciously tothe surface of the quartzite granule. This vehicle, of course,-is burnedof! and forms no permanent part of the finished fused glaze.

Th glaze mixture, including the finely divided glaze and the oil vehiclein which it is suspended and the mineral granules, are charged into amixer such as that commonly used for mixing concrete either separatelyor together. This batch is then mixed until the glaze suspension isdistributed uniformly over the surface of the granules.

The thus coated granules are then removed from the mixer and charged ina furnace which may be either of the batch type, that is, one in whicheach batch is introduced, heat treated, and then removed, or may be ofthe continuous type rotary furnace, or may be a kiln or furnace in whichno agitation takes place. A further alternative may be to leave thebatch in the first mixer and apply gas or oil heat thereto, as desired.

In the firing step the oil burns off, leaving the glaze in a uniformcoating over the surface of the quartz granules and, upon continuing thefiring, the glaze melts down to a substantially continuous film coveringthe individual granules, this coating being fused to the surface of thegranule and the heat being applied to a degree sufficient to produce aphysical and/or chemical reaction between the glaze and the surface ofthe granule but not to such an extent as to render the granule itselfmolten.

The period of time which the object is fired will depend on the kind ofbase granule em ployed, and the time is so arranged in respect to thetemperature and degree of agitation employed that the material comes outof the kiln or rotary furnace in a granular condition and needs nofurther crushing.

The glaze or vitreous enamel which I may employ in this process may beof the raw type, or may be a mixture of the desired componentspreviously fused together and ground, which latter is known as a frittedglaze. In the case of the fritted glaze the color pigment may be addedeither in the fused mixture or in the subsequent operation in oil.

One formula which I disclose as an example may be as follows formanufacturing green roofing granules:

FORMULA I Parts by weight Feldspar 30 Borax 32 Red lead 25 Sodiumnitrate 6 Whiting (calcium carbonate) l3 Chromium oxide 14 Cobalt oxide1 I In some instances it may be desired to increase theamount ofchromium oxide in the above formula, but the foregoing formula hasserved to produce a satisfactory color glazes This glazing material inan oil vehicle when mixed with the granules to be coated and fired-in arotary furnace, may be subjected to a temperature of approximately 1600F. i. e. about 8'70 C. At first, when the batch is introduced into thekiln, the oil bond in which the glaze is suspended is burned off,and, atthe end of this burning process, the glaze begins to fuse. The colorhaving been previously evenly distributed in the glaze, upon thecompletion of the fusing, a smooth even coating is accomplished, both asto glaze coating and color intensity, The temperature may then beincreased slightly beyond the fusing temperature of the glaze andmaintained for a predetermined time, which insures an even coating andthe desired extent of physical and/or' chemical reaction between thebase granule and the glaze. Either oxidizing or reducing conditions maybe maintained as occasion demands, by control of the firing step of theprocess, particularly if oil or gas are used, thus insuring good bondingand permanence of the coating and colon. With the method describedherein, I find that I am enabled to produce a color glaze having adesired strength, and variability of color in different degrees asdesired, and a permanence of color not heretofore accomplished by any ofthe methods known to me. i

The raw glaze heretofore referred to, and one which permits ofconsiderable reduction in the cost of manufacture, is one in which theraw materials are not melted together and subsequently ground, but inwhich these materials are added as separate products mixed physicallyand ground together.

FORMULA II Raw glaze formula Parts by weight Borax 30 Whiting 12 Silicafines 20 Chrome oxide 7 Sodium dichromate 14 An alternative formula isas follows:

Reverting to the formula set out herein for manufacturing green roofinggranules, I wish to point out that I have varied the percentage of greenpigment, namely chromium oxide,- and produced the intensity of colorhereinbefore referred to, which I consider to be new in this art. Theapproximate percentage of chromium oxide to the glaze is preferably 16%%where the color content of previous conventional glazes known to me isnot in excess of usually much less. For this reason, my finished articleis thus possessed of the characteristic of a more intense, and,therefore, more lasting color and, further, that if, in the manufactureof the colored granules, some chemical reaction takes place which wouldtend to affect the tone or depth of the coloring, the added quantity ofpigment will offset such tendency to affect the ultimate color andprevent injurious effects in the finished color coating.

An example of a formula for, this be asfollows:

purpose may FoRMuLA IV Redlea I 70 Borax 45 Silica 25 Chromium oxide 26ver-resistance and non-blooming characteristics of the colored ceramiccoating is due to an important degree to the ratios of ingredientsemployed, including a restricted mol ratio of boric oxide (B203) tototal basic metal oxides, of the order of, 0.6 or less, as illustratedby the above examples. By the employment of quartzite as the preferredbase granule I find there is less tendency of the base granulephysically or chemically reacting to an extent sufficient to injuriouslyafiect the color of the finished coating.

This is of considerable importance, inasmuch as it has been found thatwith some materials employed as the base granule, when subjected to hightemperatures necessary to fuse the glaze thereon, the base granulesupplies material to react with the glaze coating which has a disastrouseffect on the final color, i. e., if the base granule contains a higherpercentage of manganese it will be almost impossible to add enoughchrome oxide to the glaze to arrive at a desirable green granule. Theseundesirable features are eliminated by my invention.

In the foregoing I have described one of the processes for producing agreen glaze mineral material, and the steps of this process are, first,the step of mixing the glaze with oil to form a paste, the second stepto coat quartz or quartzite or other mineral substance mentioned withthis paste by mixing in a concrete mixer, and the third step to chargethe mixture into a furnace and fire by the batch process, with orwithout agitation.

, An alternative, as hereinbefore pointed out, may

be to agitate the mixture during the heating and fusing step, ifdesired. In this process a good distribution of glaze and color isobtained in the preliminary mix, that is, in the first step; agitationis not essental to the production of a good coatin by the step ofheating; and, furthermore, a relatively short process time may berequired where a fritted glaze is employed so that the glaze needs onlyto be melted down in situ and no additional time is needed for coatingand maturing it.

I now will proceed to describe several alternative methods formanufacturing colored coated mineral material, one of which processes isas follows:

The glaze employed in this process is conveniently a fritted lead glaze,and the process may be of a continuous nature.

The steps are to Wet the mineral, such as quartz or quartzite, in amixer with a small amount of sodium silicate solution, and, whenthoroughly mixed, the dry powdered glaze is introduced. This causes thepowdered glaze to adhere to the sodium silicate. and the mixture thusproduced of glaze and silicate immediately sets to a hard, dry, adherentfilm on the surface of the mineral. This mixture is then fedcontinuously into a rotary furnace and the heat applied to the mixtureof glaze, quartzite and sodium silicate reduces the glaze to a moltenstate, thus eventuall coating the individual granules with an evencoating.

This method has the attendant advantages of economy of the continuousoperation, and the fact that sodium silicate when mixed in the properproportions withv the fritted glaze sets to a relatively hard cement,and the quartz, when coated with this cement film, is a free flowingmass which handles nicely in the screw feeder and will flow freelythrough a feed pipe. Although the sodium silicate glaze film is notuniform enough to produce a uniform and as desirable a coating, whenfired, without the step of agitation during firing, the bonding of theglaze granule is an aid to producing a fixed and even coating.Furthermore, this process cuts down the hazard due to dust in themixture.

A further alternative method is to employ raw glaze in a continuousprocess together with sodium silicate, and the steps are as follows:

First the raw glaze is' made up by mixing together the various powderedraw materials in a concrete mixer with a charge of iron' balls or in aball mill. From this point on, the procedure is to mix the granularmaterial with a small amount of sodium silicate solution for wetting thesame. When thoroughly mixed, the glaze is added and adheres to thesodium silicate and immediately sets to a hard, dry, adherent film onthe surface of the quartz. It must be kept in mind that the quartz iswet by the sodium sili cate solution, and the glaze is added and themixing is continued until the glaze is uniformly distributed over thequartz. Thi mixture is then fed continuously into a rotary furnac andheated in a manner suilicient to cause the glaze to melt and flow ineach instance on each quartzite. granule and form a uniform coatingthereon.

It will be understood that in'this method the amount of sodium silicatesolution is not enough to be considered as a material part of the glaze,and its major purpose is to settle or control dust thrown oil by thegranule charge.

The advantages of the last mentioned method are that the raw glaze is alow cost material and that a good relatively dry mix is obtained whichhandles easily in a screw feeder. The sodium silicate cements the glazeto the granules in such a way as to prevent ring formation due to dustin the furnace, and the process has the advantage of economy inoperation.

By the ring formation referred to is meant the depositing of dustparticles ofglaze in the early stages of firing, which eventually chokesup the opening in the furnace almost completely. Reference is made tothis elsewhere in this description.

A further altematlve method of coating the granule involves theemployment of a raw glaze formula appearing elsewhere in thisdescription, and sodium dichromate is used as one of the ingredients.The quartz or quartzite granules are wetted with water in a concretemixer or the like, and the dry powdered glaze is added thereto until themass is homogeneous. The thus coated quartzite granules are then fedcontinuously into a rotary furnace and heat is' applied for fusing theglazing material on to the individual granules.

The advantages of this method are that the sodium dichromate melting ata low temperature (325' c., i. e. abbut 617 F.), acts as a vehicle tobond the glaze constituents to the granule and help their spreading inthe early stages of the process, and, as a result, better coating isobtained. Sodium dichromate can be substituted to take the place of atleast 50% of the chrome oxide used for producing the color, and thusreduces the cost of the coloring agent, and, if handled carefully andproperly, will produce a better color development than the chrome oxide.Owing to the wet mixing, a minimum amount of dust is thrown oil and thusthe hazard from this standpoint is minimized. This method develops ringformation in the furnace, but this ring formation can be materiallyreduced by putting sufilcient water in the mix.

A still further alternative form of the method described herein may betermed a continuous wet mix and is generally applicable to green colorsonly. The raw glaze contains no sodium dichromate in this instance, butthe sodium dichromate is dissolved in water and then the quartz iswetted with dichromate solution.

The steps are as follows:

A raw glaze mixture is made up; sodium dichromate is then dissolved inwater. The quartzite is then, in a mixer, wetted with the dichromatesolution. Thereupon the glaze is added'to the wet quartz, and the entiremass mixed until homogeneous. Subsequently and continuously with themixing, the homogeneous mass of quartzite granules and glaze are fedcontinuously into the rotary furnace and, while agitating the mass, heatis applied for fusing the glaze mixture on to the individual granules.

This sodium dichromate minimizes the dichromate dust hazard and by themixing of the dichromate in water gives a good dispersion of thedichromate and thus a more even coating. The glaze mix does not cake,and, at the same time, a better coating is obtained and the cost of theproduct'is reduced by virtue of the fact that the sodium dichromate issubstituted for at least 50% of the chrome oxide used as the colorproducing medium.

The function of agitation as applied to the foregoing process is that abetter spreading of the molten glaze over the surface of the quartzitegranules is accomplished by agitation, and, in the case where raw glazesare employed, the agitation serves as a means for thoroughlyhomogenizing the glaze film itself. Furthermore, hat transfer isfacilitated during the agitation s p.

While in some of the processes described herein the quartz and glaze aresubjected to a premixing before feeding into the furnace, and incontaining sodium dichromate in which the process is a continuous wetprocess. I find that this process is generally applicable to productionof the green colored granule only.

The steps of the process are asgfbllows:

The raw glaze is made up by mixing together the materials such as setout in the raw glaze mixing operation and have then been fired withthose which haye out agitation are inferior to been 'fired duringagitation. t

I have found that ,in the actual production of the. coated granule it isdesirable to obtain a greater'degre'e of agitation than that ordinarily.

obtained in a rotar'yfurnace, and I- findfit desirable to rotate thefurnace at a higher rate,

of speed than formerly norinallyiused. I have also found that there arelower limits of speed, of rotation of thefurnace which, if the speed is.reducedbeyond these lower limits, produces an unsatisfactory coating.

I find that for my present purposes the limit of speed of rotation liesat a pointbetween land 2 R. P. M. ofcourse. thisrrate of speed.'is

purely arbitrary, as with different types and.

sizes of furnaces the speed of rotation will have to be governed as bestsuited to the particular batch to be produced. The base for the rawglaze as disclosed herein consists of a mixture of borax, whiting, and.

quartz fines,.and, of course, it will be understood that the compositionof this base may be varied.

considerably. To the above base may be added any of the commonly knownceramics, coloring agents such as chromium oxide, for green, cobaltoxide for blue, iron oxide for red, etc. For green, as hereinabovepointed out, I prefer. to replace part of the chromium oxide by sodiumdichromate, inasmuch as the latter is lower in cost and has theprocessing advantages already pointed out. Sodium dichromate melts atabout 320" C., i. e. 608 F., and the molten salt acts as a bond to .holdthe glaze constituents to the granule during the preliminary'stages offiring before theglaze proper becomes molten, and this is particularlyimportant in a raw glaze .con-, taining borax, for the reason that boraxtends to puff up or intumesce upon heating. This intumescence is verylight and powdery and much of it is blown out'o-f the furnace and lost.Sodium dichromate performs the function of preventing this intumescenceand makes for a better coated product. There are other materials such assodium nitrate, potassium nitrate, and barium hydrate having. thisdesirable property, that is, preventing intumescence and which areapplicable to colors other than green. When the glaze tends to powderoff in the early stages of firing, there is a tendency for a deposit'tolruild up on the walls of the furnace which, if not controlled withindesirable limits, will eventually choke up the opening, and hamper thefree fiow of the material through the furnace. This is what I havetermed ringed formation, and I find that the use of the materials sodiumdichromate, sodium nitrate, etc., hereinabove mentioned, reduces thering formation.

I'desire to point out that the use of water in the preliminary mixing,while not absolutely' essential, has the advantages of 1) minimizing thedust hazard during the mixing operation; (2) causing the glazeconstituents to adhere slightly to the granules, thus preventingseparation of the glaze from the quartz in the feeding equipmentj (3)the water in the mix helps to minimize the formation of deposits in thefurnace; and (4) when sodium dichromate is used in the mix, this is mosteasily handled as a water solution.

I find it convenient in using water not to make up a slurry of the glazeconstituents and water as some of the prior processes known to me nowdo, but rather to add them seperately to the mixer. This is done in allcases except where sodium dichromate is dissolved in the water, and

.I also desire to point out that in most cases I do not employ enoughwater to make an actual .slurry of thin enough consistency to spread onthe granules in a mixer.

While 'a satisfactory coated product may 'result by allowing thegranules to be subjected to the furnace treatment for about 30minutes',--I have found that'it is advantageous to allow the granules tobe subjected to heat for a period of ninety minutes or more, and thattheadvantages gained thereby are that the finished product has sodiumdichromate, a low'fusing salt, and sodium nitrate, potassium nitrate andother salts of similar properties in the firing process permits thefunctioning of these elements as a bonding or'spreading agent in thepreliminary stages of firing. Th glaze itself begins to melt in therange of 600 to 700 c. The low fusing salts melt at 300 to 400 C. oreven lower if water is present, and act as a vehicle for the 'rest'ofthe glaze constituents, carrying them to the granules andfacilitatingtheir adherence, at the same time preventing their lossthrough the agitation of the furnace until these constituents become hotenough to melt, e. g. 650 to 1000 C., and sometimes as high as 1200"v C.(about 2200 F.), and thus remain on the granule.

I thus definitely divide thefiring of the prodnot into substantially twostages, that is to'say,

(1) the interval from the charging of the furnace to the time at whichthe glaze fuses; and (2) the interval from the time of fusing of theglaze to the time of discharge of the completely coated product. Itappears, therefore, that the function of the low melting salts referredto ap plies only in the firststage of heating. However, agitation of themass in the furnace applies to both stages of heating, the latterheating stage being termed the maturing stageof the glaze.

From the foregoing I find that the time in the maturing stage is adefinitely measurable factor which affects the quality of the material.I have found that this time may range from ten to fifty minutes. For afritted glaze ten minutes is sufficient to produce a good weatheringproduct. For raw glaze I find that a greater time is desirable, thoughnot absolutely necessary, with a limit which I have found to bedesirably fixed at fifty minutes. The temperature at the end of thematuring time is. of course, at or above the fusing point of the glaze.

From the foregoing it will be observed that the components entering intothe reaction for the production of the glazes, which have a high degreeof resistance to atmospheric deterioration, include the employment of anoxygen compound of boron which reacts under heat with the othercomponents to form a weather-resisting borate or boro-silicate glaze.The oxygen compound of boron may, therefore, be the readily availableform containing the boron-oxygen radical, i. e. boric oxide (B203).Important specific compositional. phases of the present inventioninvolves heat reacting an alkali metal oxide bearing material, e. g.NazO, and often also a divalent metal oxide bearing material, 6. g. ZnO,PbO and/or 02.0, in the coating composition comprising boric and also asiliceous material assilicannes. 'l'hemol ratiooftheboric to the basicmetal oxide content of the ii i eoatingshouldberestricted'and, as illmtrated herdnabove, ratios of about 0.6, 055 or less (butabove 0.1or02)-are advantageousin.

securing coatings of weather-resistant glaae character. Ibrmnlae I andIV have ratios less than 0.8, i. e. approximately 0.55, while FormulaeII and m have ratios slightly above 0.6 but less than 0.7.

Where aluminum or alumina (m) is present a reactive borate, upon thesurfaces of the base granule by intergranular contact of the basegranules during the mixing under heat treatment at a temperature above325 C. in the presence of an adhesive vehicle containing sodiumdichromate, to form a smooth paste of plastic character tenaciouslyadhering to the base granule and continuing the mixing to provide aunimtheeoatingingrcdienmaswherefel mmetc f are employed. the finishedcoating may comprise (e. g. in addition to borates and/orbore-silicates) and/or aluminates ,of such mehls as zine-lead, etc.,and/or of sodium or horic acid, are commonly employed as. the source ofbut oxide, particularly because the same readily becmne intimatelyadmixed with such ad-v vantageous reactants as sodium dichromate, theinvention is of course'n'ot so limited. Boric oxide may be added assuch, or boric oxide and-a divalmt metal oxide may-be added together, asa similar compound, for example in the form of colunanlte, which is anaturally occurring cal-- where silica lines are employed, as in theabove formulae, and coating temperatures of theorder of 1600 1". (e. 8.'-l0 00 C.) areemployed,

the resulting coating comprises completely fused material andsuperficially or partly fused -maj 1. In the process of producingcolored granular material by aillxing to a refractory siliceous basegranule a stable, substantially permanent color bearing inorganic bond,the steps which in-.. clude homogenizing glaze forming materials uponthe surfaces of the base granule by intergranular contact of the basegranules during the mixing under heat treatment at a temperature of atleast 325' C. in the presence of an adhesive vehicle containing sodiumdichromate, to form a smooth paste of plastic character tenaciouslyadhering to the base granule and continuing the mixing to provide auniform coating of said hanogenized paste of viscous material, and thencmtinuing the heating and agitation to distribute and fuse the glazematerials on the surfacesoi the base granule and to produce chrome oxidefrom said dichromate.

2. The process as described in claim 1 further characterized in that thecoated granules are raised to a temperature of at least about 870 C.during the said heating and agitation.

3. In the process of producing colored granular material by aillxing toa refractory siliceous base water-soluble e. g. borax and/or formcoating of said homogeniud paste of viscous material, and thencontinuing the heating and agitation to distribute and fuse the glazematerials on the surfaces of the base granule and to produce chromeoxide from said dichromate.

. .4. In the process of producing colored granular. material by to arefractory siliceous base granule a stable, substantially permanentcolor, bearing inorganic bond, the steps which include homogenizingglaze forming materials, including a boric oxide, upon the surfaces ofthe base granule by intergranular contact of the granules during themixing under heat treatment at a temperature of at least about 870C. inthe presence of an adhesive vehicle containing sodium dichromate, toform a smooth paste of plastic character tenaciously adhering --to thebase granule and continuing the mixing to provide a-uniform coating ofsaid homogenized paste of viscous material, and then continuing theheating and agitation to distribute and fuse the glaze materials on thesurfaces of the base granule and to produce chrome oxide from saiddichromate.

- I 5. In the process of producing colored granular material by afllxingto a refractory siliceous base granule a stable, substantially permanentcolor bearing inorganic bond, the steps which include homogenizing glazeforming materials.

including a reactive borate, upon the surfaces of the basegranules byintergranular contact of the base granules during the mixing under heattreatment at a temperature above 325 C.

in the presence of an adhesive vehicle comprising an alkali metaldichromate, to form a smooth paste of plastic character tenaciouslyadhering to the base granule and continuing the mixing to provide auniform coating of said homogenized paste of viscous material, and thencontinuing the heating and agitation to distribute and fuse the glazeforming materials on the surfaces of the base granules and to producechrome oxide from said dichromate, the said paste of plastic characterhaving a mol ratio of B20: to the basic metal oxide content of saidglaze forming composition of not more than approximately 0.63.

6. Colored, surface coated glazed granules made in accordance with theprocess claimed in claim 3.

'7. Colored, surface coated glazed granules made in accordance with theprocess claimed in claim 1.

8. As a new article of manufacture, artificially colored decorativeroofing granules comprising granule a stable, substantially permanentcolor bearing inorganic bond, the steps which include hom glaze formingmaterials, including dense, refractory, quartzitic base granules coatedwith a fused weather-resistant glaze coating prepared in situ onsurfaces of the granules from a vitreous glaze composition comprisingsiliceous materiaL-an alkali metal dichromate and boric oxide, the molratio of B20: to the basic metal oxide content of the glaze coatingbeing 0.2-0.6, said glaze coating being substantially free ofwater-soluble alkali material.

9. As a new article of manufacture, artificially colored coateddecorative roofing granules comprising dense refractory base granulescoated with a fused weather-resistant glaze coating of a bright, greencolor prepared in situ on surfaces of the granules from vitreous glazecomposition comprising siliceous material, sodium dichromate and borax,said glaze coating comprising a basic metal boro-silicate, the mol ratioof B20: to the basic metal oxide content of saidcoating being notgreater than 0.6, but being at least as great as about 0.2, said glazecoating being substantially free of water-soluble alkali material.

10. As a new article of manufacture, artificially colored decorativeroofing granules comprising dense refractory base granules coated with afused weather-resistant glaze coating of bright, green color prepared byheating in situ on surfaces of the granules a glaze-forming compositionincluding siliceous material, basic metal oxide material comprisingsodium dichromate, and a water-soluble borate, the mol ratio of B203 tothe basic metal oxide content of said glaze-forming composition beingnot greater than approximately 0.6, said glaze coating beingsubstantially free of water-soluble alkali material.

11. As a new article of manufacture, artificially colored decorativeroofing granules comprising refractory base granules coated with a fusedWeather-resistant glaze coating prepared by heating in situ on surfacesof the granules a glaze-forming composition including siliceousmaterial, a borate, and basic metal oxide bearing material comprisingsodium dichromate and also a material bearing at least one divalentmetal oxide from the group consisting of ZnO, CaO, PhD and BaO, the molratio of B20: to the basic metal oxide content of said glaze coatingbeing from 0.2 to 0.6, said glaze coating consisting substantially of aboro-silicate of basic metals, said basic metals comprising sodium andat least one of the aforesaid divalent metals. with a chromium pigmentdisseminated therein as a coloring agent, said glaze coating beingsubstantially free of water-soluble alkali material.

12. As a new article of manufacture, artificially colored decorativeroofing granules comprising dense, refractory, quartzitic base granulescoated with a fused weather-resistant non-blooming glaze coating ofgreen color prepared by heating a glaze composition in situ on surfacesof the granules, said coating comprising a basic metal boro-silicateglaze having a mol ratio of B20: to the basic metal oxide content ofsaid coating of 0.2-0.6, said glaze coating being substantially free ofwater-soluble alkali material and having a green chromium oxidedisseminated therein as a coloring agent.

13. Glazed artificially colored granules of the class described, eachindividual particle of which comprises a base having a dense andsubstansurface coating of a plurality of superficially and partly fusedglaze producing materials in particle form in a film comprising saidglaze producing materials in a completely fused condition, said filmcomprising a borate of an alkali metal and a divalent metal and beingsubstantially free of water-soluble alkali material.

15. Roofing granules, each individual .granule of which comprises a basehaving a dense surface, a surface coating on the base formed ofsuperficially and partly fused glaze forming materials in particle formbonded and interfused to said base by a film of a portion of said glazeforming materials in a fused condition, said film comprising aboro-silicate of an alkali metal and being substantially free ofwater-soluble alkali .material. a

16. Roofing granules, each individual granule of which comprises a basehaving a dense surface, a surface coating on the base formed ofsuperficially and partly fused glaze forming materials in particle formbonded and interfused 'to said base by a film of a portion of said glazeforming materials in a fused condition, said fused film being preparedby heating in situ on surfaces of the granules a vitreous glazecomposition comprising siliceous material, sodium dichromate and boricoxide, said surface coating being substantially free of water-solublealkali material.

17. Glazed artificially colored granules of the class described, eachindividual particle of which comprises a base having a dense andsubstantially non-porous surface, and a weather-resistant surfacecoating of a plurality of superficially and partly fused glaze producingmaterials in particle form in a film comprising said glaze producingmaterials in a completely fused condition, said film comprising basicmetal borosilicate, said basic metal comprising an alkali metal and themol ratio of B20: to the basic metal oxide content of said film beingnot greater than 0.6, and said film being substantially free ofwater-soluble alkali material.

18. A composite sheet body for roofing and like purposes comprising aplastic stratum and an adherent surfacing for the same consisting ofcolored coated granular material, individual granules of which are asdefined in claim 9.

19. A composite sheet body for roofing and like purposes comprising aplastic stratum and an adherent surfacing for the same consisting ofcolored coated granular material, individual granules of which are asdefined in claim 12.

20. A composite sheet body for roofing and like purposes comprising aplastic stratum and an adherent surfacing for the same consisting ofcolored coated granular material, individual granules of which are asdefined in claim 10.

21. As a new article of manufacture, a roofing sheet material comprisinga bituminous coated base having a mineralizing layer or surfacing ofweather-resisting coated granules as defined in claim 13 adhering to thebituminous coating.

22. A composite sheet body for roofing and like purposes comprising aplastic stratum and an adherent surfacing for the same consisting ofcolored coated granular material, individual granules of which are asdefined in claim 16.

GEORGE W. SW ENSON.

